If you aren't happy, why aren't you?
Chances are it is because you want something which you do not have, objects or conditions. This is probably not a good enough reason and a review of your situation and perceptions may be in order.
You may have seen video of children in very poor countries laughing and playing, unconcerned that they should have more to be happy about. They are happy because they are playing, because they have their friends and family, and some food to eat that day. Everyone has the right to be happy, and if they can be in their situation, shouldn't you?
"Happiness consists more in small conveniences of pleasures that occur every day, than in great pieces of good fortune that happen but seldom to a man in the course of his life." - Ben Franklin (1706-1790)
Happiness is inside us, in our minds, in our thinking. It is not external material things or experiences, but the enjoyment of our thoughts and feelings. This is good because our thoughts, and therefore our happiness, are up to us.
Start by reviewing the things that you have to be happy about and dwell on these. Don't overlook the little or basic things that you take for granted.
Make a habit of substituting unhappy thoughts with happy thoughts. Whenever an unfavorable picture enters your mind, eject it and replace it with a pleasant one. We all have some nice experiences to recollect and there are usually little pleasures around us most of the time. You can even imagine enjoyable experiences, and it will have a similar positive effect.
"Happiness is like a butterfly which, when pursued, is always beyond our grasp, but, if you will sit down quietly, may alight upon you." - Nathaniel Hawthorne (1804-1864)
Establish good principles and conduct yourself according to these principles. Love, or at least be tolerant of, your fellow humans. Don't strive too hard for that which, in the end, will not make you happier.